Magnetic bounce eliminator



Nov. 10, 1970 H. H. BOWER, JR

MAGNETIC BOUNCE ELIMINATOR Filed Dec. 22, 1967 INVENTOR H.H.B0WEE,JR.

ATTORNEY United States Patent 3,538,648 MAGNETIC BOUNCE ELIMINATOR Hadley H. Bower, In, Oklahoma City, Okla., asslgnor to Western Electric Company, Incorporated, New

York, N.Y., a corporation of New York Filed Dec. 22, 1967, Ser. No. 692,997

Int. Cl. B24b 17/02 US. CI. 51-99 2 Claims ABSTRACT OF THE DISCLOSURE An apparatus for grinding a helical groove along the body of a carbon-deposited resistor, wherein the grinding wheel is mounted on one lever and having a second, cam-controlled lever engageable therewith to raise and lower the grinding wheel into and out of engagement with the resistor; including means for preventing separation and bounce between the two levers, using magnetic attraction between the levers to hold them together yet permitting separation of the levers under a force greater than their magnetic attraction.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to mechanical linkages and more particularly to apparatus for preventing bounce or separation between two levers and is applicable but not limited to use on a machine for performing a machining operation on a workpiece, such as a tool for cutting or a wheel for grinding helical grooves in depositedcarbon resistors.

Description of the prior art In the manufacture of deposited carbon resistors, a grinding wheel is normally employed to cut a helical groove through the carbon film that is deposited over a ceramic insulating tube. Prior to the start of grinding, the rapidly-spinning grinding wheel is held away from the resistor by a cam-operated lever system that controls the lateral positioning of the grinding wheel. After the resistor is in the desired grinding position, the control cam operates the lever system to allow the grinding wheel to come into contact with the resistor body. Difficulties have been experienced in that the grinding wheel, upon its initial engagement with the resistor body, has a tendency to bounce away from the resistor.

It is an object of the present invention to prevent bounce between a grinding wheel and its associated workpiece as the grinding wheel is brought into engagement with the workpiece.

It is another object of the present invention to prevent separation and bounce between two elements of a lever system.

Still another object of the present invention is to control the motion of a lever system until the load carried by the lever system exceeds a predetermined value.

SUMMARY OF THE INVENTION In accordance with the present invention separation at the connection between a pair of levers is inhibited by forming at least part of one of the levers of magnetizable material and including means in the other lever, positioned adjacent the magnetizable material in the one lever, for conducting a magnetic flux and for causing the magnetic flux to pass through the magnetizable material in the one lever, thereby developing a force tending to hold the two levers together.

BRIEF DESCRIPTION OF THE DRAWING The present invention and its objects will be more fully 3,538,648 Patented Nov. 10, 1970 ice The present invention is described in connection with a resistor helixing machine. In the manufacture of carbon-deposited resistors, a resistive coating of carbon is deposited onto the outside surface of an insulating ceramic cylinder. In order to increase the electrical resistance from one end of the carbon coating to the other, a helical groove is machined in the periphery of the cyl inder, through the carbon coating thereon. This increases the electrical resistance of the carbon resistor by constraining the electricity to flow in a narrow helical path around and along the resistive coating. Such resistors are shown in greater detail, together with a prior art apparatus for cutting the helical groove therein, in US. Pat. No. 2,884,746 granted to A. F. Rus et al. on May 5, 1959. In the Rus et al. patent the resistor is moved into engagement with a stationary grinding wheel, and the resistance of the resistor is monitored during the grinding operation to stop grinding when the resistance reaches a predetermined value.

The copending application of N. J. 'Mandonas et al. Ser. No. 551,598 filed on May 20, 1966, now Pat. No. 3,408,896 also shows a prior art apparatus for cutting helical grooves in resistors, using a movable grinding wheel.

'In machining a carbon-deposited resistor, the resistor body is suspended between a pair of rotatable, translatable chucks. The resistor and the grinding wheel are brought into engagement with each other and the resistor is rotated. Either the resistor or the grinding wheel can be translated axially to trace a helical path along the surface of the rotating resistor.

Referring now to the drawing, a resistor 11 is shown in position between two rotatable chucks 13 and 15 for rotation and translation therewith. The leftmost chuck 13 is spring biased (not shown) to the right and is selectively pulled to the left by a solenoid 17 to release the resistor from the chucks. The axial position of the resistor 11 is controlled by the chuck 15 in response to a cam 19 operating through a cam follower 20, a bell crank 21, and a spline collar 23. Rotation of the chuck 15 at a constant speed is accomplished by a timing motor 25 that rotates a spline shaft 27 that is in engagement with the spline collar 23, to permit a reliable rotational connection between the timing motor 25 and the chuck 15. Therefore, the timing motor determines the rotational speed of the resistor 11.

A small toothed pulley 29 is also mounted on the spline shaft 27 for rotation therewith and drives a larger toothed pulley 31 by means of a toothed belt 33. The more-slowly-rotating toothed pulley 31 drives a cam shaft 35 on which the cam 19 is mounted. It can be seen that the shape of the helical cut made on the resistor 11 is determined by the ratio of the pulleys 29 and 31 and by the profile of the cam 19 that slowly moves the chuck 15 to the right against a bias spring 36, as the shaft 35 is rotated by the timing motor 25. The profile of the cam .19 is made to move the chuck 15 and the resistor 1-1 steadily to the right in order to cut a groove having substantially a constant helix angle onto the periphery of the resistor 11.

A step 37 in the cam 19 causes the chuck 15 and the resistor 11 to move rapidly to the left under the urging of the spring 36 after they have reached their rightmost position. Therefore, the chuck 15 moves slowly longitudinally while it advances to the right and then experiences a rapid longitudinal movement to return the chuck to its leftmost position to begin another helical motion to the right.

Another cam 39 is mounted on the cam shaft 35 for rotation therewith and has a profile which engages the movable contact of a contact pair 41, closing the contacts 41 slightly before the step 37 on the cam 19 reaches the cam follower 20 connected to the bell crank 21 which causes the chuck 15 to return rapidly to the left. When the contacts 41 close, they complete a circuit from a power source 43 (which may be a battery, a 60- cycle electrical plug, or any other suitable source of electrical energy) through the solenoid 17, operating the solenoid 17 and causing it to withdraw the chuck 13 to its leftmost position out of engagement with the resistor 11 in order to release the resistor.

The profile of the cam 39 is arranged to open the contacts 41 in order to allow the chuck 13 to move to the right under the urging of its bias spring (not shown) when the cam 19 advances the chuck 15 to its leftmost position into engagement with a new resistor 11 supplied either manually or by a resistor feedirg means (not shown) which forms no part of the press at invention. Therefore, when a resistor has been machined, the

chuck .13 moves to the left, releasing the resistor from the grip of the chucks. A new resistor then drops or is manually inserted into place and the chucks move together (not necessarily simultaneously) to grip it and to hold it at a point determined by the profile of the cam 19.

When the new resistor 11 is in position between the chucks 13 and 15, a cam 51 mounted for rotation with the cam shaft pushes against the rightmost end of a control lever 53 tending to rotate the lever clockwise about its pivot 55 against a tensile spring 57. An arm 61 on the control lever 53 bears against the leftmost end of a main lever 63 and permits the lever 63 also to rotate clockwise about its pivot 65. A grinding wheel 71 is rotatably mounted at the righthand end of the main lever 63 and is firmly connected by means of a rotatable shaft 68 to a drive pulley 67 that is driven by a double pulley 69 through a drive belt 73. The double pulley 69 is rotatably mounted on the pivot so that rotation of the main lever 63 about its pivot will not shorten or lengthen the center distance between the double pulley 69 and the driven pulley 67. The double pulley 69 is driven through a driving belt from a driving pulley 77 that is in turn continuously rotated by a motor 79. The bearing friction produced by the manner in which the grinding Wheel 71 and the driven pulley 67 are mounted to the main lever 63 provides sufficient torque to urge the main lever 63 in the clockwise direction about its pivot 65 under control of the control lever 53.

As the main lever 63 rotates clockwise about its pivot 65, the grinding wheel 71 engages the resistor 11; and the arm 61 on the control lever 53 separates from the leftmost end of the main lever 63, permitting the friction torque at the bearing of the rotating grinding wheel and the pulley 67 on the main lever 63 and the grinding torque to be used to develop the grinding force that urges the grinding wheel 71 into engagement with the resistor 11.

It has been found that the grinding wheel 71 has a tendency to bounce away from the resistor 11 upon its initial impact therewith, resulting in erratic initial grinding of the helical groove in the resistor 11. This could result in a discontinuous helical groove that could cause an area on the resistor surface to be locally overheated when the resistor is put into service in an electrical circuit. In addition, since no measure is made of the resistance across the resistor 11 in order to determine the extent to which the helical groove is ground therein, this erratic grinding at the initial moment of impact between the grinding wheel 71 and the resistor 11 results in an unacceptably wide range of resistance variation from resistor to resistor.

In order to prevent this initial bounce of the grinding wheel 71, the arm 6.1 of the control lever 53 is made of a magnetically permeable material; and a permanent magnet 81 is mounted in the leftmost end of the main lever 63 at the point of contact between the two levers and causes a magnetic flux to flow between the poles of the permanent magnet 81, through the arm 61. Therefore, as the control lever 53 is rotated in the clockwise direction by the control cam 51, the magnetic flux gives rise to a magnetic attraction between the arm 61 and the permanent magnet 81 in the main lever 63, tending to hold the levers together; and the initial force with which the grinding wheel is applied to the resistor 11 is determined not only by the friction with which the grinding wheel and the driven pulley are mounted to the main lever 63 but also by the magnetic attraction between the permanent magnet 81 and the permeable arm 61. This magnetic attraction tends to prevent the grinding wheel 71 from initially bouncing away from the surface of the resistor 1.1 and causes the grinding wheel actually to dig more deeply into the resistor until the control cam 51 applies sufficient upward force on the arm 61 to exceed the magnetic attraction between it and the permanent magnet 81, and the levers slowly separate with a steadilydiminishing magnetic attraction as the gap between the arm 61 and the magnet 81 is lengthened. The permanent magnet attraction between the two levers also assures that the grinding wheel experiences a motion according to the profile of the cam 51 irrespective of any variations in the friction of the drive system of the grinding wheel 71.

Continued rotation of the chucks 15 and 13 and of the cam shaft 35 causes the grinding wheel 71 to cut a helical groove through the resistive coating on the resistor 11 until a low point 83 on the control cam 5.1 permits the tensile spring 57 to rotate the control lever 53 counterclockwise about its pivot 55, bringing the arm 61 into engagement with the main lever 63. Further rotation of the control lever 53 rotates the main lever 63 counterclockwise about its pivot 65, removing the grinding wheel 71 from engagement with the resistor 11.

It is to be understood that the above-described arrangement is simply illustrative of the application of the principle of this invention. Numerous other arrangements may be readily devised by those skilled in the art, which will embody the principles of the invention and fall within the spirit and scope thereof.

What is claimed is:

1. In an aparatus for machining a workpiece;

machining means for selectively engaging and performing work on the workpiece;

a first pivotal lever for supporting said machining means;

a second cam-controlled and pivotal lever normally positioned to be in contacting relationship with said first lever, said first lever moving said machining means into engagement with a workpiece in response to the cam-controlled movement imparted to said first lever by said second lever, and

magnetic means secured to at least one of said first and second levers for normally preventing undesired separation and bounce between said first and second levers and between said machining means and a given workpiece intended to be in engagement therewith, said magnetic means establishing a predetermined magnetic attractive force between said first and second levers sufiicient to normally main tain said levers in contacting relationship, at least during the period of actual machining of a workpiece, but said magnetic means allowing said levers to separate should a force be exerted therebetween which is opposite to and of a magnitude sufficient to overcome the magnetic attractive force.

2. An apparatus according to claim 1 wherein said magnetic means comprises a permanent magnet as an integral part of one of the levers.

References Cited UNITED STATES PATENTS 1,561,197 11/1925 Wenderhold. 2,213,014 8/1940 Owen.

6 2,226,287 12/1940 Miller.

2,446,776 8/1948 Matson. 2,893,551 7/1959 Pirwitz 2698 OTHELL M. SIMPSON, Primary Examiner US. Cl. X.R. 

